Letterman himself said during his monologue: "It’s been great...and the network has been great, but I’m retiring." He added a heartfelt thanks:

I just want to reiterate my thanks for the support form the network, all of the people who have worked here, all of the people in the theater, all the people on the staff, everybody at home, thank you very much.

In October of 2013, Letterman extended his contract for an additional two years, keeping him on the air until 2015. Said Letterman at the time, "[CBS president] Les [Moonves] and I had a lengthy discussion, and we both agreed that I needed a little more time to fully run the show into the ground."

There's no specific timetable for his departure, but Letterman classified it as "sometime in the not too distant future."

It's unclear what prompted Letterman's exit, but it's possible that once he outlived rival Jay Leno, he thought it was time to be done. Even without Leno, Letterman has faced stiff, and younger, competition from newcomers like Jimmy Kimmel, Jimmy Fallon and Seth Meyers.

Letterman remains the longest running late-night talk-show host in history, clocking in at 31 years and counting.

UPDATE 5:50 P.M. - Letterman's YouTube page has posted video of Dave's announcement, seen in the player above.